1. Field of the Invention
The present invention relates to a wavelength monitor, a wavelength lockable laser diode (hereafter denoted as LD) implementing with the wavelength locker, and a method for locking the emission wavelength of a tunable LD.
2. Related Background Art
Kimoto et al. disclosed a semiconductor laser module including a tunable distributed feedback (hereafter denoted as DFB) LD and a wavelength locker in a single package (Furukawa Technical Report 112, July, 2003, pp. 1 to 4). The wavelength locker disclosed therein had two photodiodes (hereafter denoted as PD), one of which detected a portion of back facet light directly from the DFB-LD, while the other of which detected another portion of the back facet light through an etalon filter. The emission wavelength of the DFB-LD may be tuned through these two detections.
Recent wavelength division multiplexing (hereafter denoted as WDM) system has ruled a span between nearest two grid wavelengths as 50 GHz within the wavelength region of 192 to 197 THz, which corresponds to the 1550 nm band. In such a system, an optical signal source is required to control the emission wavelength further precisely and stably. The emission wavelength of an LD often fluctuates due to operating temperature and/or a long-term degradation of device performance. The wavelength locker for such an LD is inevitable in the WDM system.
Conventional wavelength lockers have been implemented with an optical component having a periodic transmission spectrum against wavelengths. An etalon filter is one of typical components shown in such periodic transmission spectrum. The period between the transmission maxima of the etalon filter matches with the span of the grid wavelengths of the WDM system.
However, the wavelength locker described above leaves a subject that the locking performance may be available only within a narrow wavelength range. That is, when the emission wavelength of the
LD shifts more than one period of the transmission spectrum, the wavelength locker tunes the emission wavelength next to the target wavelength, which is a fatal subject when such a wavelength locker is going to be applied to the recent WDM system.
Moreover, recent optical apparatus further requests to make the housing or package thereof as compact as possible. When the wavelength locker is realized by discrete components of the DFB-LD, the etalon filter, and PDs; these devices are arranged independently and coupled with, for instance, a condenser lens to obtain a satisfactory coupling condition, which inevitably enlarges the size of the housing/package.